1. Field of the Invention
The present invention relates to a piezoelectric oscillator and, more particularly, to a piezoelectric oscillator that uses a MOS construction type capacitance element.
2. Description of the Related Art
Various forms of circuits have hitherto been proposed and put to practical use as an oscillator that uses a piezoelectric resonator that is represented by a crystal resonator. They are being used in a wide variety of electronic appliances, for example, as signal sources of portable telephones, computers, etc.
On the other hand, in each of such oscillators, for achieving various objects that include making adjustments of the frequency at the time of the manufacture, realizing the channel-frequency-adjusting function or AFC (automatic frequency control) function, etc. and that further include making compensations for the frequency in connection with the temperatures, conforming the oscillator to a large number of channel frequencies, etc., it is indispensable to use a variable-capacitance element.
As a circuit that can realize such objects, there is generally used a circuit that is illustrated in, for example, FIG. 6.
The oscillation circuit illustrated in the figure is an ordinary circuit of the crystal oscillator using an inverter amplifier. In this oscillation circuit, between an input and an output of an inverter amplifier 101 there is inserted a parallel circuit comprised of a crystal resonator 102 and a feedback resistor R1. Also, between the input and the output thereof and the ground there are inserted a capacitor C1 and a capacitor C2, respectively. Simultaneously, to either one of the capacitor C1 and the capacitor C2 (in this example to the capacitor C1) there is connected a variable-capacitance diode D1 serving as a variable-capacitance element. And, a cathode of the variable-capacitance diode D1 and a control terminal Vcont are connected to each other via a resistor R2 for preventing the flow of direct currents.
Because the operation of this oscillation circuit is well known, it is thought unnecessary to newly give an explanation of it. Briefly explaining, however, in this oscillation circuit, according to a direct current voltage that is applied to the control terminal Vcont, the capacitance value of the variable-capacitance diode D1 varies. For this reason, by controlling this control voltage, it is possible to perform various kinds of adjustments of the frequency, for example, AFC and so forth as mentioned above.
On the other hand, in view of the recent demands for miniaturization, and reduction in the power consumption, of various kinds of electronic appliances, there has been also a demand for conversion of the above-described oscillator to an IC version.
However, in a case where converting the oscillation circuit including the variable-capacitance diode D1, such as that illustrated in FIG. 6, to an IC version, this diode cannot but be formed using a process step different from that for forming other semiconductor circuits. Therefore, this diode has been an obstacle to cheaply converting the oscillation circuit to an IC version.
Namely, the variable-capacitance diode D1 which is a bipolar type of semiconductor must be formed using a process step separate from that for forming the inverter amplifier 101 which is generally a C-MOS type of semiconductor. Therefore, the process for such formations is not only complex but the IC product was high in cost.
On the other hand, as a variable-capacitance element suited to conversion to IC version, there is known a MOS construction type capacitance element, the utilization of that has been in expectation.
As a crystal oscillator using such a MOS construction type capacitance element, there is the one that is disclosed, for example, in Japanese Patent Application Laid-Open No. 10-13155 entitled xe2x80x9cCrystal Resonator with Frequency-Adjusting Functionxe2x80x9d.
This crystal oscillator is constructed as follows. As illustrated in FIG. 7, between an input terminal and an output terminal of an inverter amplifier 101 there is inserted a parallel circuit comprised of a crystal resonator 102 and a feedback resistor R1. Additionally, a capacitor C2 is connected to the output terminal of the inverter amplifier 101, and a MOS construction type capacitance element 103 is connected to the input terminal of the inverter amplifier. And, simultaneously, an electric-charge injection terminal TI of the MOS construction type capacitance element 103 and a control terminal Vcont are connected to each other.
As the MOS construction type capacitance element 103, although it is only a mere one example, there is known the one illustrated in FIG. 8. Namely, in this element 103, a positive or negative voltage is applied to the control terminal Vcont by using the N type substrate as a basis to thereby cause the flow of a tunnel current through the interior of SiO2 to thereby cause electrons to inject into or come out of a floating electrode 104.
Namely, for example, in a case where having applied a positive voltage to the control terminal Vcont, electrons flow out of the floating electrode 104. Therefore, the thickness of a depletion layer 105 situated near the floating electrode 104 becomes narrow, with the result that with a decrease in that thickness the depletion layer capacitance increases.
Also, in a case where having applied a negative voltage to the control terminal Vcont, the operation reverse to that mentioned above occurs. So, an explanation thereof is omitted.
However, as will be explained below, fundamentally, the MOS construction type capacitance element can have its capacitance value varied over a wide range only with use of a positive power supply or negative power supply. For this reason, there was the drawback that almost no change in the capacitance value occurred when merely using only either a positive, or a negative, single-polarity power supply alone.
This will hereafter be explained in a little more detail.
FIG. 9 is a graph illustrating an example of the relationship between an inter-electrode voltage and a capacitance value of the MOS construction type capacitance element.
As clear from this figure, in this example, when the terminal-to-terminal voltage varies within a range of from xe2x88x921.5V to +0.5V including therein 0V therebetween, the capacitance value changes over a range covering approximately 80 pF.
However, on the other hand, in the crystal oscillator, whereas the range within that the frequency is variable need generally be wide, it is more preferable that the capacitance value be gently varied with respect to the change in the control voltage than that sharply in order to perform high-precision frequency control. For this reason, there has been a demand for a variable-capacitance element whose capacitance value varies over a wide range of control voltage.
Accordingly, in the case of the crystal oscillator such as that illustrated in FIG. 7, in order to obtain a wide range of variable capacitance with use of the MOS construction type capacitance element 103, it is necessary to use control voltage sources for applying both positive and negative voltages to the control terminal Vcont. Therefore, there was the problem that the construction of the system making control of the frequency became complex.
The present invention has been made in order to solve the above-described problems and has an object to provide a small-sized piezoelectric oscillator which, while using a MOS construction type capacitance element suited to conversion to an IC version, enables obtaining a wide range of changes in the variable capacitance even with use of either a positive, or a negative, single-polarity power supply, and which facilitates the frequency control.
To attain the above object, according to the first aspect of the invention, there is provided a piezoelectric oscillator wherein, in an oscillator including a piezoelectric resonator, an amplifier, and a variable-capacitance element, the variable-capacitance element is a MOS construction type capacitance element, one terminal of that is applied with an alternating current voltage, whose intermediate voltage is a V voltage, and the other terminal of that is applied with a control voltage falling within a range whose intermediate value is the voltage.
According to the second aspect of the invention, there is provided a piezoelectric oscillator wherein, in an inverter piezoelectric oscillator in which a piezoelectric resonator is connected between an input terminal and an output terminal of an inverter amplifier; and divisional capacitors C1 and C2 are connected between respective ends of the piezoelectric resonator and the ground, by inserting a MOS construction type capacitance element in series with the piezoelectric resonator, one end of the MOS construction type capacitance element is applied with a bias voltage which is the V voltage at an output end or input end of the inverter amplifier and the other end thereof has supplied thereto a control voltage that varies within a range whose intermediate value is the V voltage.
According to the third aspect of the invention, there is provided a piezoelectric oscillator wherein, in an inverter piezoelectric oscillator in which a piezoelectric resonator is connected between an input terminal and an output terminal of an inverter amplifier; and divisional capacitors C1 and C2 are connected between respective ends of the piezoelectric resonator and the ground, two MOS construction type capacitance elements are inserted respectively on both sides of the piezoelectric resonator; one end of each of the MOS construction type capacitance elements is applied with an alternating current voltage, whose intermediate voltage is a V voltage; and the other end thereof is applied with a control voltage that varies within a range whose intermediate value is the V voltage.
According to the fourth aspect of the invention, there is provided a piezoelectric oscillator wherein, in an inverter oscillator in which a piezoelectric element is connected to an input or output end of an inverter amplifier; and divisional capacitors C1 and C2 are connected between respective ends of the piezoelectric element and the ground, a MOS construction type capacitance element is inserted between the piezoelectric resonator and an input end of the inverter amplifier or between the piezoelectric resonator and an output end of the inverter amplifier, a control voltage Vcont is applied to the terminal on a connection-to-piezoelectric resonator side of the MOS construction type capacitance element; and, when it is assumed that V represents the voltage that is a direct current bias voltage at the input end or output end of the inverter amplifier and that is applied to one end of the MOS construction type capacitance element, it is arranged that said voltage becomes an intermediate voltage of the control voltage Vcont.
According to the fifth aspect of the invention, there is provided a piezoelectric oscillator wherein, in an inverter oscillator in which a piezoelectric element is connected to an input or output end of an inverter amplifier; and divisional capacitors C1 and C2 are connected between respective ends of the piezoelectric element and the ground, a MOS construction type capacitance element is inserted between the piezoelectric resonator and an input end of the inverter amplifier or between the piezoelectric resonator and an output end of the inverter amplifier and a control voltage Vcont is applied to the terminal on the connection-to-piezoelectric resonator side of the MOS construction type capacitance element; a direct current circuit of a resistor and a capacitor is inserted and connected between the terminal on the inverter-amplifier side of the MOS construction type capacitance element and the input or output terminal of the inverter amplifier; and further a direct current bias voltage is applied to the terminal on the inverter-amplifier side of the MOS construction type capacitance element.
According to the sixth aspect of the invention, there is provided a piezoelectric oscillator according to the fifth aspect of the invention, wherein the amplitude level of an alternating current supplied to the MOS construction type capacitance element is adjusted according to the value of the resistance of the direct current circuit; and when it is assumed that V represents the direct current bias voltage supplied to the terminal on the inverter-amplifier side of the MOS construction type capacitance element, it is arranged that the direct current bias voltage V becomes an intermediate voltage of the control voltage Vcont.